It is known that, especially in the field of water treatment, fixed culture techniques are employed in which the ability of microorganisms to produce exopolymers enabling them to be fixed to very diverse supports in order to form biofilters is used. Although, in these applications, the properties of the biofilms are exploited beneficially to remove contaminants (nitrates, phosphates, etc.), on the other hand, the same biofilms are often undesirable, or even detrimental. Thus, in some industrial plants immersed in aqueous media, the development of microorganisms, bacteria and algae, which are deposited on the plants in the form of biofilms, constitutes real microbiological pollution and the prevention and control of the growth of these biofilms require implementing relatively sophisticated, efficient and expensive techniques, the most common of which is chlorination.
Optimization of the treatments (improving performance, removing waste, reducing costs) is related to the possibility of determining the growth rate of the biofilms in real time.
At present, methods making it possible to detect the presence of biofilms and which have been the subject of industrial development are few. In order to illustrate the prior art in this field mention may especially be made of the following publications.
U.S. Pat. Nos. 4,912,332 and 5,337,376 relate to a method of detecting microbiological pollution using optical fibres. This known technique is based on measuring transmitted and absorbed light in order to monitor the formation of the biofilms. It has the drawback of being difficult to use in pipes.
U.S. Pat. No. 5,576,481 describes a method of detecting on-line biofilms based on measuring the heat transfer coefficient. The change in the latter is directly related to the development of the biofilm.
An apparatus making it possible to identify the biofilms and to control the use of biodispersants, which uses the conventional method of measuring the adenosine triphosphate is known by the brand “Bioscan”. This apparatus has in particular the following drawbacks: on the one hand, the measurement depends on the bacterial cells sampled in the medium studied and not on the biofilm itself and, on the other hand, it is complicated and expensive.
Another sensor for monitoring the biofilm, called “BIoGEORGE” is known, which consists of two stainless steel electrodes, mounted on a body also made of stainless steel, and a data control and acquisition system. The measurement is based on analysing the change in current flowing between the two electrodes, after interruption by a galvanostatic prepolarization. This change is correlated to the presence of the biofilm and to the type of corrosion products on the surface of the electrodes. Given that the interaction between the biofilm and the stainless steels is very complex, detection of the biofilm alone seems very difficult with this sensor and, in addition, the measurements can only be carried out in seawater.
The “BioX” sensor records the value of the galvanic coupling current between a stainless steel and a copper alloy which is associated with the growth phase of the biofilm.
The “BioGuard” sensor, based on the electrochemical detection of the catalysis for the reduction of oxygen by bacteria, makes it possible to monitor the first steps in the formation of the biofilm.
B. N. Stokes et al (“Developments in on-line fouling and corrosion surveillance” published in “Microbiologically influenced corrosion testing” in 1994 by J. R. Keans and B. Little, Philadelphia, USA) have developed a device enabling corrosion and fouling to be monitored having the form of a miniaturized heat exchanger in which the corrosion is monitored by a measurement using an ammeter with zero resistance, a measurement of the electrochemical noise (current/potential) and a measurement of the linear polarization resistance, while fouling is detected by measuring the heat transfer coefficient. The use of these four electrochemical techniques makes the system complex and not very adaptable on site.
G. Salvago et al (“Biofilm monitoring and on-line control: 20-month experience in seawater” published in 1994 in “Microbiol Corrosion” by the European Federation of Corrosion) have studied the behaviour of stainless steels and of aluminium brasses exposed to seawater. The combination of the heat transfer resistance and of the electrochemical measurements under cathodic polarization enables the growth of a biofilm on the inner surface of tubes to be monitored. The various techniques used in such a system make it complex.
Experience demonstrates that all the currently known biofilm detectors have the drawback of being complex in their use. The present invention therefore set itself the objective of providing a biofilm sensor making it possible to measure the thickness of the biofilm and which can be used in various media (seawater, freshwater, water from industrial processes, etc.).
According to the present invention, the ability of a given medium to develop a biofilm when travelling around a circuit is assessed by electrochemical means by using a sensor based on the principle of the electrochemical cell with three electrodes.
Firstly, and according to a first aspect, this invention aims to provide a method of determining the thickness of a biofilm developing on a support immersed in an aqueous medium where such a biofilm is developing, characterized in that it consists in:                a) continuously circulating the said medium in an electrochemical cell comprising a reference electrode, an auxiliary electrode and at least one working electrode;        b) interrupting the circulation of the said medium and isolating the cell;        c) introducing an electrochemical tracer in the medium present in the cell;        d) circulating the medium+tracer solution in the said cell so as to direct a jet of the said solution perpendicularly to the working electrode;        e) measuring and recording the value of the limiting current for reducing the said tracer, as a function of the hydrodynamic conditions at the surface of the working electrode, and        f) calculating the thickness of the porous biofilm layer at the surface of the said electrode by applying the Koutecky-Levich equation, based on analysing the transport of matter through the porous layer which relates the value of the limiting current for reducing the tracer to the flowrate of the solution.        
With regard to the Koutecky-Levich equation, reference may be made to the article by D. Herbert-Guillou et al published in Electrochimica Acta, 1999, Vol. 4, No. 7, page 1067.